1. Field of the Invention
The present invention relates to suppressing noise in electronic circuits, especially in electronic communications circuitry, and more particularly to suppressing EMI (Electro-Magnet Interference) noise signals that propagate through the air.
2. Background Information
Noise in electronic circuits is described typically as propagating via common signal returns or ground paths, common power supplies, and EMI through the air.
Well known protective measures include: ground planes where signal returns have very low impedances resulting in little voltage noise; filters on power supply connections that impede signals from propagating between circuits via the power supply; and careful layout rules and shielding to reduce EMI. For EMI the prior art teaches shielding and laying out circuits and circuit modules so that mutual inductances, capacitive paths and EMI generation and reception are minimized.
In modern electronics, the electronic circuits are digital clocks (used herein synonymously with oscillators) and logic circuits that run computers and communications systems (PC's, hand held devices, etc.) at very high speeds. These high frequency signals also have sharp edges, and, together, create EMI noise that may exhibit RF (radio frequency) harmonics in the selectivity ranges of RF receivers with enough energy to adversely affect the RF receiver's performance.
FIG. 1 illustrates prior art circuit design of a CLK CKT (clock circuit) and a logic circuit, LOGIC CKT A, each with a separate filter to the power supply, VDD, and a ground plane 2. These circuits represent virtually any electronic circuitry. VDD also has C1 and C2 bypass capacitors that present a low impedance to signal frequencies over a broad frequency spectrum. The separate filters each have a series inductor, L1 and L2, respectively, together with a bypass capacitor, C3 and C4, respectively, that filter signals traveling to or from the CLK CKT and the LOGIC CKT A via the VDD connection. EMI signals 5, however, may propagate via the air as shown whenever high speed frequencies or fast edges occur. These EMI signals may include RF harmonics that the RF antenna 4 will accept. These harmonics, received by the CKTRY B, increase noise and thus decrease the receiver's sensitivity.
In hand held electronics, shielding is often not a practical option, and layout considerations cannot eliminate EMI generation and their RF harmonics from coupling to RF receivers. Regardless of the care taken in the designs, such RF receivers suffer from increased noise and decreased sensitivity due to EMI.